1. Field of the Invention
The present invention relates to a glass suitable for use as a disk for recording medium such as magnetic disk.
2. Description of the Related Art
In the field of magnetic disk, technical development is being made rapidly toward increasing the recording density and transfer rate. Nowadays, it is urgent to develop a high-speed disk for rapid transfer. Hence, there is a demand for a disk material with a high specific rigidity which will not vibrate during high-speed running. Conventional aluminum disks have a specific rigidity of 26.7 (Young's modulus [72 GPa] divided by density [2.7 g/cm.sup.3 ]). It is said that aluminum disks need more than twice that specific rigidity if they are to be used at high speeds of 10000 rpm. The only way to double the specific rigidity of aluminum disks is to combine aluminum with ceramics. This is not practical from the standpoint of production cost.
On the other hand, glass disks (2.5-inch in size) are attracting attention because it is easy to increase their specific rigidity. This object is achieved by heating glass at an adequate temperature, thereby causing a crystalline phase with a high Young's modulus to separate out. The resulting glass ceramic has a high Young's modulus. For example, Japanese Patent Laid-open Nos. 329440/1994, 111024/1996, and 221747/1996 disclose a process for causing lithium dioxide crystals and .alpha.-quartz crystals to separate out. And Japanese Patent Laid-open No. 77531/1998 discloses a process for causing spinal crystals to separate out, thereby increasing Young's modulus to 109-144 GPa and specific rigidity to 36-47.
The disadvantage of the disclosed technology is that crystallization increases the specific rigidity of glass but it also gives rise to a composite structure (composed of the hard crystalline phase and the soft glass phase). Such a composite structure produces minute steps at the time of polishing, making it difficult to obtain a super-mirror required of disks.
There is a way to increase the specific rigidity of glass per se by incorporation with a rare earth metal which improves Young's modulus of glass. The disadvantage of this technology is that incorporation with a rare earth metal increases not only Young's modulus but also specific gravity, with the result that the specific rigidity of glass does not increase as expected.
One way to increase Young's modulus without remarkably increasing the specific gravity of glass is to replace nitrogen with oxygen in glass, thereby producing oxynitride glass. Japanese Patent Laid-open No. 1327/1998 discloses using oxynitride glass as a disk substrate. It indicates in its example that the glass has an extremely high Young's modulus of 139-185 GPa and a comparatively low specific gravity of 2.9-3.4 g/cm.sup.3, with the specific rigidity being as high as 47-55. However, the composition disclosed in its example suggests that the glass contains nitrogen in such a large amount as to form inhomogeneous glass, with the glass partly crystallized. As in the case of glass ceramics mentioned above, such glass gives rise to steps at the time of polishing, making it difficult to obtain a super-mirror. Another disadvantage of glass with microcrystals which have separated out is a slow polishing rate. This leads to a high production cost because time required for fabrication accounts for a large portion of production cost, particularly in the case of mirror-finishing. Glass containing microcrystals increases in fracture toughness to such an extent that abrasive grains do not readily produce minute cracks during polishing. This is the reason why the polishing rate is extremely low in the case of inhomogeneous glass or crystallized glass containing microcrystals.